When laying a surface on a pavement or road, a heated asphalt paving material is spread onto a prepared bed and spread out while still hot. The paving material is then compacted so that a hard surface is formed upon cooling. Paving vehicles generally have a compacting device known as a screed, having a substantially flat sole plate, mounted on the rear of the vehicle so that the paving material is compacted as the vehicle moves forward and pulls the device over the paving material. The screed is usually made from steel or other heavy material and is mounted on pivotable arms which allow the screed to move up and down in the vertical direction over the paving material. Conventional screeds often have vibrators mounted on the sole plate and the resulting vibration of the sole plate, together with the weight of the screed, serves to compact the heated paving material.
Conventional screeds often also include a tamper which is a bar mounted forwards of the sole plate and driven by means of an eccentric drive. The tamper effectively pre-compacts the paving material before compacting by means of the sole plate occurs.
The sole plate and tamper must be heated, otherwise the paving material begins to harden upon coming into contact with the screed, and adheres to the screed causing a build up of material on the screed which causes an undesirable drag on the paving vehicle, and leaves marks or open textures on the hardened paving material. In particular, it is important that the tamper is heated, which can pose difficulties as the tamper bar is constantly reciprocating. The sole plate must also be heated, as the sole plate can become bound to the tamper, and hinder the reciprocating motion, if the heated paving material binds to the sole plate upon cooling.
Known methods of heating the tamper include provision of a heating element in the tamper bar. However, the electrical supply to the heating element often becomes faulty due to the repeated motion of the tamper.
U.S. Pat. No. 6,124,580 provides a possible solution to this problem by the application of a high frequency electric current to a coil connected with the sole plate so as to induce eddy currents in the tamper in order to heat the tamper bar as the tamper moves with respect to the coil. However, that solution requires the use of a high frequency current, and the tamper bar will not be heated until there is relative motion between the tamper bar and the coil.
The present invention is directed to overcoming one or more of the problems set forth above.